1. Field of the Invention
This invention relates to a magnetic head for a video tape recorder. More particularly, it relates to the use of fusion glass for laminating ferrite substrates and inserting fusion glass in a track width adjustment groove by way of gap bonding.
2. Related Art
In the field of magnetic recording, a laminated metal film head, comprising a lamination of plural layers of thin magnetic metal films with the interposition of insulating layers, has been examined as a magnetic head for high density magnetic recording having excellent high-frequency characteristics.
Such a magnetic head is shown for example in FIG. 29, in which plural magnetic metal thin films of reduced film thicknesses are stacked via insulating films and sandwiched in their entirety by a pair of magnetic core substrates 103, 104 and 105, 106 to form magnetic core halves 107, 108 which abut each other at the end faces of the stacked metal films 101, 102 and are fused to each other by fusion glass 109. In general, since the combined film thicknesses of the stacked metal films are set so as to be equal to a track width TW of a magnetic gap g, the magnetic core substrates 103, 104; 105, 106 are formed of a non-magnetic material, such as crystallized glass or ceramics.
However, if, with such magnetic head, the track width TW is reduced for achieving a higher recording density, the head efficiency is lowered, as shown in FIG. 30, due to the decreased core cross-sectional area of the laminated metal films 101, 102.
For this reason, it has been proposed to employ an oxide magnetic material, such as ferrite, in the magnetic core substrates 103, 104; 105, 106 for maintaining the cross-sectional area of the core to preventing the head efficiency from being decreased.
In such case, it is possible with the magnetic head employing the ferrite substrate to prevent the head playback output from being lowered due to the deterioration in the head efficiency as compared to the magnetic head employing the non-magnetic substrate, as shown in FIGS. 31 to 33. Such effect becomes outstanding if the track width is reduced to an extremely small value of, for example 5 .mu.m.
A magnetic head in which ferrite is used in the magnetic core substrates 103, 104; 105, 106 is produced by the following sequence of operations.
First, the portion on a major surface 110a of a ferrite substrate 110 which is to become a magnetic gap and its adjacent area is machined to form a substantially U-shaped groove 111 along the entire length of the substrate 110, as shown in FIG. 34.
A glass 112 as a non-magnetic material is inserted in the groove 111, as shown in FIG. 35.
The portion of the glass which has overflown the groove 111 is ground off smooth until the major surface 110a of the ferrite substrate 110 is exposed in its entirety, and subsequently the ground surface is machined to a mirror surface finish.
Magnetic metal thin films having reduced film thicknesses are stacked on the major surface 110a and finished to a mirror surface, for forming a laminated metal film 113.
A glass film 114 is formed on the laminated metal film 113, such as by sputtering, so that a ferrite substrate as later explained may be subsequently bonded thereon.
Then, as shown in FIG. 38, another ferrite substrate 115, prepared in a similar manner with the same shape, is stacked on the glass film 114, so that the ferrite substrates 110, 115 are bonded to each other by the glass film 114 to form a unitary block 116.
The block 116 is then severed along the centerline of the groove 116 filled with the glass 112 into magnetic core half-blocks 117, 118.
Then, as shown in FIG. 39, winding grooves 119, 120 for placing coils, not shown, and glass grooves 121, 122, are formed in the magnetic core half-blocks 117, 118.
The surfaces of these magnetic core half-blocks 117, 118 which later form the magnetic gap are machined to a mirror surface finish and a gap film is formed on the magnetic gap forming surfaces.
Then, as shown in FIG. 40, the magnetic core half-blocks 117, 118 are bonded to each other by a fusion glass 123 to form a unitary block.
Then, auxiliary winding grooves 124, 125 are formed in the unitary block and a surface along which slides a magnetic recording medium is ground to a cylindrical surface.
Finally, the assembly is cut to a required size to complete a magnetic head shown in FIG. 41.
With the above-described production method, not only is the process from FIG. 34 to FIG. 36 for controlling the track width complex, but a step difference is produced between the level of the glass 112 inserted in the groove 111 formed in the ferrite substrate 110 and that of the ferrite substrate 110 shown in FIG. 36, thus adversely affecting the characteristics of a sputtered magnetic metal film.
That is, when mirror-surface finishing a forming surface 110a for the laminated metal film 113, a step difference H is generated due to difference in hardness between the glass and the ferrite substrate. The step difference may be on the order of 20 to 30 nm. If the thin magnetic metal film is laminated via an insulating film on the surface presenting the step difference H, the magnetic film continuity is lost and deteriorates the magnetic characteristics of the laminated metal film 113.
According to a conventional practice for overcoming such deficiency, a plurality of thin magnetic metal thin films are laminated via insulating layers on the ferrite substrate not formed with a groove and a glass film is formed thereon. After bonding to a ferrite substrate, not formed with the groove, the assembly is split into two along a surface which is to become a magnetic gap, and a track width adjustment groove is machined in the ferrite substrate.
After forming winding grooves and glass grooves in the ferrite substrate, the ferrite substrates formed with the winding grooves and the glass grooves are abutted to each other with a gap film in-between, and fusion glass is inserted into the track width adjustment groove. The resulting assembly is ground to a cylinder which is then cut into pre-set chips for completing magnetic heads.
Although the production process may be simplified with the above-described method for producing the magnetic heads to prevent deterioration of magnetic properties of the laminated metal film, the laminated metal film tends to be eroded by the fused glass inserted into the track width adjustment groove which decreases the effective track width and lowers the playback output.
Thus it may be contemplated to form protective films 137, 138 formed of a material which is substantially not eroded by the fusion glass 136, inserted into the track width adjustment grooves 134, 135 formed in the magnetic core halves 132, 133, produced by stacking the ferrite substrates 129, 131 formed with the laminated metal films 126, 127 on the ferrite substrates 128, 130 with the interposition of glass films 140, 141, such as metal films of Cr, Ti, Zr or Ta, or metal oxides, such as Cr.sub.2 O.sub.3, TiO.sub.2, ZrO.sub.2, Ta.sub.2 O.sub.5, on the laminated metal films 126, 127 contacted with the fusion glass 136, as shown in FIG. 44.
However, since the material of the protective films 137, 138 exhibits a high abrasion resistance during the sliding of the tape as compared to the laminated metal films 126, 127, fusion glass 136 or the ferrite substrates 128, 129, 130 and 131, the protective films 137, 138 are protruded with the head wear as shown in FIG. 45. If the magnetic tape 139 is run in this state to reproduce the magnetic signals, a spacing loss corresponding to the protrusion S of the protective films 137, 138 is produced to lower the playback output, as shown in FIG. 46. If the magnetic head is produced by the above method, the fusion glass 136 inserted in the fluid state in the track width adjustment grooves 134, 135 during fusion tends to be cracked to lower the gap length accuracy significantly. On the other hand, if the heating temperature employed when laminating the ferrite substrates 128, 129, 130, 131 after formation of the laminated metal films 126, 127 is raised excessively, not only are the film characteristics of the laminated metal films 126, 127 lowered significantly, but the ferrite substrates 129, 131 undergo cracking due to the difference in the thermal expansion coefficients between the laminated metal films 126, 127 and the ferrite substrates 129, 131.
Also, as for the glass films 140, 141, the strength of ferrite is lowered at the contact area between the ferrite substrate and the glass film due to the reaction between the ferrite and the glass during heating intended for laminating the ferrite substrates, thus leading to a fracture 142 as shown in FIG. 47 in the course of the various machining operations following the lamination of the substrates. Besides, bubbles 143 are generated within the glass films due to the reaction between the ferrite and the glass and powders worn off from the magnetic recording medium, such as the magnetic tape, tend to be deposited in the bubbles 143 to cause the clogging of the magnetic head.